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Symbiosis & Normal Microbiota

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Last revised: Monday, April 17, 2000
Ch. 28 in Prescott et al, Microbiology, 4th Ed.
Note: These notes are provided as a guide to topics the instructor hopes to cover during lecture. Actual coverage will always differ somewhat from what is printed here. These notes are not a substitute for the actual lecture!
Copyright 2000. Thomas M. Terry

Types of Symbiosis

Examples of symbiotic relationships

  1. Symbiotic Nitrogen Fixation
    • Mutualistic symbiosis between bacteria (Rhizobium species) and roots of leguminous plants (alfalfa, clover, vetch, peas, beans, etc.) --> root nodules
    • Allows growth in nitrogen-poor soils
    • Bacteria provide ammonia by nitrogen fixation, create special anaerobic microenvironments in which this can happen. Plants provide nutrients and shelter.
    • When bacteria are removed from nodule, cannot carry out nitrogen fixation. Some of needed gene products come from plant genes.
    • Note: there are non-symbiotic nitrogen-fixing bacteria, e.g. Azotobacter. Also other types of symbionts, e.g. Frankia that live in Alder roots, create nodules.
  2. Ruminants & Resident microbes
    • Ruminants (R) are herbivorous animals with four-chambered stomach = rumen.
    • R eat grasses containing mainly cellulose, but lack enzymes to digest cellulose.
    • Bacteria and Protists in rumen produce cellulases, hydrolyze cellulose to sugar which is then fermented.
    • Products include:
      methane (from methanogens)
      organic acids (acetate, propionate, butyrate).
    • Acids are adsorbed by R into bloodstream, provide source of energy.
    • Methane must be released by belching, ~2 liters/min. Disease "bloat" when cows can't belch.
    • Microbial population totally anaerobic, achieves highest density of bacteria (up to 1012 cells/ml).
    • Cellulose digestion is slow process. Animals regurgitate rumen contents back to mouth to facilitate breakdown, "chewing cud".
  3. Syntrophic symbioses
      Cross-feeding symbioses: metabolic products of one organism are required by another
    • Ex: H2 is produced by a variety of anaerobic fermentations, as a way of getting rid of electrons from NADH. This same H2 is required by anaerobic methanogens in order to carry out anerobic respiration:
      4H2 + CO2 ---> CH4 + 2 H2O (Methanogens)
    • In absence of H2-producing bacteria, methanogens can't grow.
    • Q. for further thought: how would you grow methanogens in a laboratory?
  4. Lichen symbiosis
    • Lichens are associations of fungus (host) with photosynthetic alga or cyanobacteria (symbiont).

      crustose lichens, commonly found on rocks and trees
    • Resulting symbiotic organisms can grow attached to rocks, tree trunks, other unlikely habitats. Fungus (ectosymbiont) provides minerals by releasing lichen acids that dissolve substrate, release small amounts of P, S, other minerals, and obtains water from air. The endosymbiont carries out photosynthesis, converts CO2 to organic matter to feed itself and fungus host.
    • Partners in lichen symbiosis can be separated and will grow as individual fungi or alga/cyanobacteria.
    • For further information, visit "Lichens" Web page, from "Natural Perspective".

Gnotobiotic Animals

Normal Microbiota of Humans



Bacterial Disease Case Study: Dental caries and Streptococcus mutans
  • Teeth in skulls from Europeans prior to the 1500's showed remarkably well-preserved teeth. Once sucrose, a dissacharide from cane sugar, was introduced into the European diet, teeth deteriorated quickly and tooth decay became a widespread disease.
  • The bacterium Streptococcus mutans (along with S. sobrinus) play an important role. In the process of breaking sucrose down into its component sugars, glucose and fructose, S. mutans polymerizes all the glucose units into a dextran polysaccharide, using the enzyme dextransucrase.

    Light micrograph of Strep. mutans. From Dr. Timothy Paustian, University of Wisconsin-Madison
  • Since S. mutans uses lactic acid fermentation exclusively as its catabolic pathway, enormous quantities of dextran are produced when sucrose is present. This accumulates as a gooey polysaccharide matrix which initiates the formation of dental plaque.
  • As plaque forms, other bacteria colonize and small food particles are trapped. Rapid bacterial metabolism causes anaerobic environments; lactic and other acids are produced, attacking tooth enamel and causing tooth decay (dental caries).
  • Acids also attack surrounding tissues, producing gingivitis (periodontal disease).

GI tract

Genitourinary tract

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